W. Stephen tait, ph.D. Chief Science Officer & principal Consultant, pair O Docs professionals, LLC Corrosion Corner Can storage tests accurately predict actual corrosion? mass that sustains corrosion growth through the materials and generate enough corrosion to be seen either with the unaided eye or with a light microscope. We’ve seen numerous instances where pitting corrosion that perforates aerosol containers in one year was not detected until around 6–9 months of storage testing. Insufficient numbers of samples Limited storage test room space often restricts the number of spray packages that are put on test. The result is small limited numbers of samples for examination at each pull-time (examination time). There are two things wrong with a small number of samples for a given examination time: 1) low statistical confidence and 2) you probably won’t have enough samples to find the corrosion that will eventually cause package failure, such as leaking. For example, it is unlikely that you would find corrosion if 10% of the packages will leak and you examine only three containers. Unrealistic test parameters Sometimes storage test aerosol containers are stored in several orientations, such as upright, inverted and on their sides. The upright and inverted orientations provide the same results. Evaluating corrosion of containers on their side is appropriate only when the commercial containers will be stored by consumers on their sides. Storing containers at high temperatures for multiple months also does not simulate actual container storage by customers and consumers. The length for storing containers at higher temperatures should be based on the number of hours per year that packages are stored in warm areas. Also, the test temperature should reflect realistic high temperatures for the various areas where products are warehoused and sold. Not evaluating the effect of variability on corrosion Variability is a fact of life that could contribute to or cause spray package corrosion. Storage room space limitations also often prevent evaluating how variability affects spray package corrosion. Hello, everyone. Unexpected corrosion occurs from a number of corrosion storage test technical practices: • Using high temperatures to accelerate package material corrosion • Short storage times • Insufficient numbers of samples • Unrealistic test parameters • Not evaluating the effect of variability on corrosion • Not determining the effective concentration range for corrosion inhibitors • Not examining containers from first product runs Let’s briefly discuss each. Using high temperatures to accelerate package material corrosion Last month, I discussed the corrosion myth about using high temperatures to accelerate the rate of corrosion for packaging materials. Accelerating corrosion is desirable because—if successful—the length of a storage test could be shortened and products would be introduced more quickly into the marketplace. However, the corrosion rates of package materials are not accelerated by higher temperatures. In some instances, higher temperatures cause formula ingredient degradation that contributes to or causes the corrosion of spray package materials. Accelerated corrosion test results can be obtained through use of instruments that detect corrosion and measure corrosion rates before package corrosion is massive enough to be seen. Short storage times Shortening storage test lengths is an outgrowth of using higher storage temperatures to accelerate material corrosion rates. It takes time for corrosion to initiate, increase to a critical size/ 26 Spray April 2017
SprayApril 2017
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